This invention generally relates to television receiver tuning systems and more specifically is directed to an improved UHF tuning system capable of also tuning to channel frequencies in the so-called CATV ultraband.
Indirect frequency synthesizer tuning systems are well known and generally include a phase locked loop (PLL) synthesizer as a local oscillator for generating a plurality of local oscillator frequencies with a predetermined frequency separation maintained between adjacent frequencies. With each frequency representing an individual channel, output signals of a phase comparator in the PLL are applied via a low pass filter to electronic tuning elements generally including a plurality of varactor diodes. Each varactor diode is incorporated in one of several, coupled tunable circuits such as RF input/output amplifier stages, a mixer stage, and an oscillator stage. These various tunable stages are generally designated the "tuner" with the oscillator stage generally referred to as the local oscillator.
The PLL circuit generally utilized in a conventional indirect frequency synthesis tuner includes a variable, or programmable, divider, a reference crystal oscillator, a reference divider, a phase comparator, a low pass filter and a voltage controlled local oscillator. Upon receipt of a predetermined binary signal corresponding to the selected channel, the variable divider produces a frequency-divided signal obtained by dividing the frequency of an output of the voltage controlled oscillator by a predetermined dividing ratio. The frequency-divided signal and a reference frequency signal produced by dividing the output of the reference crystal oscillator are applied to the phase comparator, and an output signal of the phase comparator is applied via a low pass filter to the varactor diodes in the above mentioned tuner circuit which includes the voltage controlled oscillator. The voltage controlled oscillator output may then be provided to a mixer where it is combined with the received signal to produce an intermediate frequency (IF) or the tuner itself may convert the voltage controlled oscillator output to an IF signal for processing in the television receiver.
Tuners which include the aforementioned varactor diodes are generally referred to as electronic tuners. The basic limitation in these circuits is related to the restricted capacitance variation in the tuned circuits and the resulting limited frequency spectrum over which these resonant circuits may be tuned. By increasing the capacitance ratio of the varactor diodes in these tuned circuits, the frequencies available are correspondingly increased. However, the internal resistance of a varactor diode increases for higher rated breakdown voltages. Thus, while a greater capacitance voltage range may be obtained by using diodes of higher voltage breakdown ratings for increasing the tuning voltage, the increased varactor diode internal resistance limits tuning circuit gain and, as a result, is counter-productive. This characteristic of varactor diodes imposes an upper limit on the tuning voltage which can be applied to these tunable circuits. Various multi-band tuners are disclosed in the prior art, most of which use automatic band switching for electronically changing the tuned circuits in response to either user channel selection or to a converter device that converts CATV signal frequencies to a common intermediate frequency, generally that of VHF channel numbers 3 or 4.
The present invention provides a tuning system which employs a single tunable circuit for tuning a television receiver over either the UHF band or the CATV ultraband by compensating for differences in the transmitted frequencies of channels in these overlapping bands.